126 



ECHINOTHURIA 



plate. Each plate is perforated by a pair of pores. It differs from EcJdnofhuria in 

 every particular. 



" The question presented to me by IMr. Elower's fossil was, whether to consider it part 

 of the envelope of a new kind of Holofhuria, or whether it might be no more than a fragment 

 of the oral disc of some great unknown Echinus. Portions of the imbricating scaly 

 armour of a Psolus had been met with when examining the fossils of the Boulder Clay 

 collected by Mr. J. Richmond, of Rothsay ; but in Psolus, while the greater part of the 

 body is clothed with fish-like scales, the ambulacra are only develof)ed on one side, form- 

 ing a creepuig disc, the scales of which are small and not imbricated. On the other hand, 

 the peristome of the largest know^n Echinite from the Chalk is less than an inch in 

 diameter ; and the largest recent Sea-urchin in the Museum has an oral disc not more 

 than two inches wide, whereas the fossil is a segment of a disc which must have been at 

 least four inches across. This objection, on the score of size, was, however, less felt, 

 because the Cyphosomas and Diademas of the Chalk have larger oral and apical orifices 

 than any other Urchins, and the character of their apical disc was unknown, being only 

 preserved in a few minute specimens of C. difficile, from Chute Farm. ]\Ioreover, there 

 were indications in the Upper Chalk of a great Biadema, of which nothing more had been 

 obtained than scattered plates and fragments of s[)ines. This species is referred to in 

 Decade V of the ' Geological Survey ' (Article " Diadema," Section C, spines tubular, annu- 

 lated). Mr. Wetherell obtained a mass of Chalk containing above one hundred fragments 

 of spines, which are hollow, striated and annulated, as in the recent B. calaniaria. From 

 the plates mingled with the spines we ascertained that the ambulacral pores presented 

 the usual characters, being arranged in single file, and a little crowded near the peristome ; 

 but many of the plates presented only their smooth inner surfaces. A smaller mass of 

 Chalk, in Mr. Wiltshire's cabinet, contains similar plates and spines, mingled with a fcAV 

 true scales and minute truncated spines like those of Echinofharia. The Diadema spines 

 were erroneously referred by Professor E. Forbes to the genus Micraster (Decade III, pi. 10, 

 fig. 15 ; bad, for they are not spiral). They are also figured by Dixon, in his 'Geology 

 of Sussex,' and described by Forbes as ''spines of a Cidaris.'' Diademas possessing spines 

 of this character are known to occur in the Upper Cretaceous strata of France ; and 

 Dr. Wright has lately obtained a small specimen from the Chloritic Marls of Dorsetshire. 

 In these the apical disc is quite small. 



" A more serious difficulty, in comparing Mr. Flower's fossil with the oral disc of any 



Echinite, was presented by the arrangement of the plates ; in the recent Echinidae 



they are all directed towards the dental orifice, but here the alternate series take opposite 

 ' dips,' the ambulacral plates overlapping one way and the others in a contrary direction. 



" Last year, while I was still hesitating about the publication of Mr. Flower's fossil, a 

 second specimen was obtained from Charlton, in Kent, by the Rev. N. Glass, wdio has 



cleared it from the matrix with great skill and patience At first sight this specimen 



would seem to solve the problem, by supplying the peristome and lantern of the same 



